Label plugin refactor (#5251)

* Add skeleton for builtin label printing plugin

* Force selection of plugin when printing labels

* Enhance LabelPrintingMixin class

- Add render_to_pdf method
- Add render_to_html method

* Enhance plugin mixin

- Add class attribute to select blocking or non-blocking printing
- Add render_to_png method
- Add default method for printing multiple labels
- Add method for offloding print job

* Simplify print_label background function

- All arguments now handled by specific plugin

* Simplify label printing API

- Simply pass data to the particular plugin
- Check result type
- Return result

* Updated sample plugin

* Working on client side code

* Cleanup

* Update sample plugin

* Add new model type

- LabelOutput model
- Stores generated label file to the database
- Makes available for download

* Update label printing plugin mixin

* Add background task to remove any old label outputs

* Open file if response contains filename

* Remove "default printer" option which does not specify a plugin

* Delete old labels after 5 days

* Remove debug statements

* Update API version

* Changed default behaviour to background printing

* Update label plugin mixin docs

* Provide default printer if none provided (legacy)

* Update unit test

* unit test updates

* Further fixes for unit tests

* unit test updates

.gitignore

@@ -40,9 +40,11 @@ inventree-demo-dataset/
 inventree-data/
 dummy_image.*
 _tmp.csv
-inventree/label.pdf
-inventree/label.png
-inventree/my_special*
+InvenTree/label.pdf
+InvenTree/label.png
+label.pdf
+label.png
+InvenTree/my_special*
 _tests*.txt
 
 # Local static and media file storage (only when running in development mode)